Thermal barrier coatings are highly advanced material systems usually applied to metallic surfaces, such as gas turbines or aero-engine parts, operating at elevated temperatures, as a form of exhaust heat management. These coatings serve to insulate components from large and prolonged heat loads by utilizing thermally insulating materials which can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface. In doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending part life by reducing oxidation and thermal fatigue.
In certain commercial applications, materials are desired that possess low thermal conductivity and low heat capacity, while fulfilling requirements of high-temperature capability and structural integrity during repeated temperature cycling and operational stresses and mechanical loads. Materials with low thermal conductivity are of interest when thermal protection is necessary or when heat loss is undesired. Materials with low heat capacity are of interest for applications in which temperature swings are encountered and when the insulation material should not significantly affect the temperature swing.
In the internal combustion engine, materials that insulate the hot combustion gas from the cold, water-cooled engine block are desired to avoid energy loss by transferring heat from the combustion gas to the cooling water. At the same time, during the intake cycle, the insulation material should cool down rapidly in order to not heat up the fuel-air mixture before ignition to avoid knocking. See Kosaka et al., “Concept of Temperature Swing Heat Insulation in Combustion Chamber Walls and Appropriate Thermophysical Properties for Heat Insulation Coat,” SAE Int. J. Engines Vol. 6, Issue 1 p. 142 (2013). For such an application, low thermal conductivity and low heat capacity is required. Furthermore, low thermal conductivity is only required at high temperatures; at low temperatures, a higher thermal conductivity is beneficial.
The existing practice utilizes ceramic thermal barrier coatings (TBCs), typically 7 wt % yttria-stabilized zirconia. TBCs have very low thermal conductivity (0.8-1.6 W/m·K at room temperature), but relatively high heat capacity (2000-2300 kJ/m3·K at room temperature). The 10-20% porosity created by the deposition method is either random between different “splats” for plasma-sprayed coatings or “feather-like” for electron beam vapor-deposited coating, as explained in Clarke and Levi, “Materials Design for the next Generation Thermal Barrier Coatings” Annu. Rev. Mater. Res. 33 pp. 383-417 (2003). Both types of cellular architecture are detrimental to structural integrity, and the brittle ceramic material causes low damage tolerance. Another disadvantage is that both cellular architectures are fairly open and gases from the outside can access many of the pores and even the underlying substrate.
Other prior art for thermal barrier materials includes thermal protection systems for space applications, such as tile for the Space Shuttle. Shuttle tiles exhibit low thermal conductivity and low heat capacity, but these materials are designed for extreme temperatures (greater than 1300° C.) and have poor mechanical properties (crush strength less than 0.5 MPa). They often cannot be reused after one flight due to changes in shape. These tiles are open cellular structures and absorb significant amount of water, which increases mass and results in damage when the water is vaporized during exposure to high temperatures.
In view of the prior art, what is needed is a thermal barrier material that possesses low heat capacity and low thermal conductivity, while at the same time, high structural integrity and robustness. Such thermal barrier materials preferably are suitable for both coatings and for bulk (freestanding) materials and parts. Good performance is desired even in thin coatings.